Vacuum wafer bonding technology provides a number of very effective techniques to produce low-cost, hermetically sealed packages for micromachined sensors and actuators. Besides the protection of the device from outside environmental stresses, the package must also provide a cavity ambient compatible with the device performance and reliability. This invention is directed to remove or alleviate the shortcomings of existing fine leak tests to assess the stability of the cavity atmosphere over long time. A new ultra-fine leak test based on quality factor (Q-factor) monitoring has been developed that has the potential to be used for in-line critical leak rate testing on wafer level before or after device singulation. With the example of a poly-silicon resonating microsensor, it is shown that the test is sensitive enough to predict the cavity pressure for a demanded device lifetime of fifteen years, independent of whether or not an integrated thin film getter is used for stabilizing the cavity vacuum and extending device lifetime.
Microsensor packaging is one of the most important and challenging technology areas (W. Reinert et al., “Vacuum Wafer Bonding Technology”, IMAPS Nordic 2004). In particular, hermetic packaging on wafer level is a key technology of many microelectromechanical systems (MEMS). The hermetic sealing protects them from harmful environmental influences, significantly increasing their reliability and lifetime. In addition some MEMS need a specific gas or pressure environment for damping of the resonating structure within the package to function as specified, see table 1. A number of technological topics are to be considered to produce hermetically sealed, micromachined devices on wafer level with controlled cavity pressures (“vacuum pressures”) ranging from 10−4 mbar to 1000 mbar.
TABLE 1Required vacuum level for different MEMS.sensor/device typevacuum levelaccelerometer300-700mbarabsolute pressure sensor1-10mbarresonator (angular rate)10−1mbarbolometer<10−4mbarRF-switch<10−4mbar
Wafer-level processes are particularly interesting for the MEMS packaging since they can reduce the fabrication costs and open up possibilities for batch processing. Various wafer level sealing technologies may be used, including wafer bonding, cavity sealing by thin-film deposition, and reactive sealing, see table 2.
TABLE 2Vacuum wafer level sealing technologies.SealingtechnologycharacteristicsWafer bondingEstablished, effective technologiesLarge cavity volumeDamping atmosphere controlGetter integration possibleRobust cap withstands moldingLimited device density on waferThin film cappingVery high device densityNo getter integration possibleVery small cavity volumeNo atmosphere controlCap may bend under molding press.Cap transferNot an established technologyDamping atmosphere controlGetter integration possibleParallel processThin caps reduce device thicknessIndividualVery large cavity volume possiblecappingCeramic caps for RF applicationsSequential placement processGetter integration possible